Ethanol consumption could lead to 60 medical conditions. Acute as well as chronic toxic effect of ethanol may ensue in irreversible organ damage (Das S. K. et. al., Indian journal of Biochemistry & Biophysics, 2010, Vol. 47, 32). The widely accepted forms of alcoholic liver diseases (ALD) are simple fatty liver (steatosis), which is reversible with abstinence, fatty liver accompanied by inflammation (steato-hepatitis) leads to scar tissue formation (fibrosis), the destruction of the normal liver structure (liver cirrhosis), which may or may not improve with abstinence and subsequently lead to liver cancer (hepatocellular carcinoma). In 2010, WHO suggests 10% of the adult population in the United States suffering from alcohol use disorders and liver cirrhosis is the 12th leading cause of death in United States (Alcohol and Health, Focus on: Alcohol and the Liver, 2010, Vol. 33, No. 1 and 2, 87). It is known that 5% of the ethyl alcohol i.e. ethanol (hereinafter alcohol), ingested by a human being is excreted unchanged while the remaining 95% is degraded to acetaldehyde. Alcohol is rapidly absorbed from the GI tract. In fasting state the peak blood alcohol concentration reaches within 30 minutes. Distribution is rapid with tissue levels approximating blood concentrations. Liver accounts for nearly 90% of alcohol metabolism the remainder is excreted through the lungs & urine. The typical adult can metabolize 7-10 g of alcohol/hour (U.S. Pat. No. 7,666,909B2).
The primary pathway of alcohol metabolism, when consumed in low to moderate amount, is mainly catalyzed in the cytoplasm of hepatocytes by alcohol dehydrogenase (ADH) to form acetaldehyde. The accumulation of NADH (excess reducing equivalents) in the liver plays a role in liver damage seen more prominently with chronic alcohol use. Acetaldehyde produced through microsomal ethanol oxidation system (MEOS) initially represents a minor pathway of ethanol oxidation probably accounting for less than 10%, of the liver capacity to oxidize ethanol.
At higher alcohol level (>100 mg/dl), MEOS is dependent on CYP450 (2E1, 1A2 & 3A4) plays significant role in alcohol metabolism using NADPH as a cofactor & O2. Catalase is especially capable of oxidizing ethanol in fasting state in the presence of hydrogen peroxide generating system. Acetaldehyde is oxidized in the liver via mitochondrial nicotinamide adenine dinucleotide (NAD+) dependent aldehyde dehydrogenase (ALDH) to acetate. Activity of ALDH is nearly 3 times lower that ADH, hence accumulation of Acetaldehyde takes place. Acetate is further metabolized to acetyl CoA and can enter min TCA cycle or synthesis fatty acids. Each of these pathway results in the formation of free radicals (like reactive oxygen species {ROS}) with concomitant changes in the cells redox state (i.e. in the ratio of NADH to NAD+ results in production of more NADH (Nicotinamide Adenine Dinucleotide (NAD+) reduced by two electrons). The cell has a limited capacity to oxidize NADH back to NAD+ in mitochondrial respiratory chain at the maximum capacity of this system, which determines the kinetics of the reaction. The redox state in relation to alcohol metabolism causes inhibition of NAD+-mediated enzyme reactions typical to the normal metabolism of the hepatocyte. The citric acid cycle is affected the most as it gets inhibited. This leads to positive NADH/NAD ratio, which is considered the most important reason for the development of alcohol-induced fatty liver. The maximum capacity of the mitochondrial respiratory chain depends on the overall level of metabolism of the body. The consequence of altered redox state includes Hypoxia (oxygen deficit cell). The other plausible pathway of alcohol induced hepatotoxicity includes excess production of pro-inflammatory cytokines by gut-endotoxin stimulated Kupffer cells. ROS is mainly generated in association with the mitochondrial electron transport system; it is also produced by CYP2E1 and by activated Kupffer cells in the liver. Both acute and chronic alcohol consumption can increase ROS production, which leads to oxidative stress through a variety of pathways mentioned above [(Zakhari, S. Alcohol Research & Health, 2006, 29, 4, 245), (Wheeler M. D. et al, Free Radical Biology & Medicine, 2001, Vol. 31, No. 12, 1544), (Koop, D. R., Alcohol Research & Health, 2006, 29, 4, 274), (U.S. Pat. No. 7,666,909B2)].
The mechanisms involved by which alcohol causes cell injury are complex and combination of several inter-related pathways. ROS react primarily with the cell membrane (tight junction becomes more permeable) and in turn leaks lipopolysaccharides (LPS), as a consequence impaired gut structural integrity. The increases in transaminase enzymes [aspartate amino-transferase (AST) and alanine aminotransferase (ALT)] indicate cellular leakage and loss of functional integrity of cell membrane (Yue et. J, 2006). Loss of cellular integrity affects hepato-biliary function leading to elevated alkaline phosphatase (ALKP) activities with concurrent increase in serum bilirubin level and decrease in the total plasma protein content. Both increases and decreases in the levels of ROS can lead to apoptosis of hepatocytes (Wheeler M. D. Alcohol Res. Health, 2003; 27, 300). For the cell to function normally, GSH is critical to protect itself against ROS generated during activity of the mitochondrial respiratory chain. Alcohol consumption rapidly depletes GSH levels; alcohol interferes with Cytochrome c to leak from the mitochondria into the cytosol, which can activate enzymes known as caspases that can trigger apoptosis.
ROS induces LPO [ROS reacting with Malondialdehyde (MDA), 4-hydroxy nonenal (HNE)] and recognized as important starting place of hepatocytes damage. Endotoxin-activated Kupffer cells affects mitochondria leading to release of ROS (hydrogen peroxide radical, hydroxyl radical, particularly superoxide radical) and several cytokines (viz., Tumour necrotic factor {TNF-α}) leading to hepatocytes necrosis and apoptosis. It has been established by clinical studies that patients with alcoholic liver disease have increased levels of the inflammatory cytokines IL-1, IL-6, and TNF-α as well as the chemokine IL-8 and other cytokines.
Alcohol might enhance the sensitivity of hepatocytes, consequently which could lead to an increased production of ROS in the mitochondria. ROS could activate a regulatory protein called nuclear factor kappa B (NFκB), which plays critical role in regulation of immune response and controls the activities of numerous genes, including those that expresses TNF-α & its receptor as well as genes encoding proteins that promote apoptosis. Thus, a vicious cycle would be established in the hepatocytes: TNF-α promotes ROS production, which in turn activates NFκB, leading to enhanced production of additional TNF-α and its receptor as well as to production of factors that promote apoptosis. This cycle eventually alters the structure of the hepatocytes, impairs their function, and can lead to hepatocyte apoptosis. TNF-α also facilitates hepatocyte regeneration by promoting the proliferation [(Wheeler M. D. Alcohol Res Health, 2003; 27, 300), (Molina P., Happel, K. I., Zhang P., Kolls J. K., Nelson S., Focus on: alcohol and the immune system. Alcohol Res. Health, 2010, 33 (1 & 2), 97)1)].
TGF-β (transforming growth factor beta) might be involved in the development of alcohol-induced liver damage, which could cause the hepatocytes to produce molecules like trans-glutaminase, cytokeratins that are normally responsible for giving the cells their shapes. In excess, these molecules are cross-linked to form microscopic structures called Mallory bodies, which are markers of alcoholic hepatitis. TGF-β can also contribute to liver damage by activating stellate cells. In a normal state, these cells primarily serve to store fat and vitamin A in the liver. When activated, stellate cells produce collagen, the major component of scar tissue it leads to the development of liver fibrosis. Alcohol might trigger the activation of TGF-β and thereby contribute to the initiation of apoptosis if this molecule enters the blood in higher concentrations (Wheeler M. D., Alcohol Res. Health, 2003; 27, 300).
Acetaldehyde or ROS with DNA or protein or protein building blocks and ROS with MDA or MAA (mixed MDA-acetaldehyde-protein adduct) or HNE etc. in the cell could form stable or unstable adduct, which could be carcinogenic, immunogenic, induce inflammatory process, damage to the mitochondria etc. [(Zakhari, S. Alcohol Research & Health, 2006, 29 (4)245); (D. Wu, Alcohol Research & Health, 2106, 27, 4, 277); (Wheeler M. D., Alcohol Res. Health, 2003; 27, 300); (Molina P., Happel K. I., Zhang P., Kolls J. K., Nelson S., Focus on: alcohol and the immune system; (Alcohol Res. Health, 2010, 33, Vol. 1 & 2, 97); (Neuman M. G., Cytokine-central factor in alcoholic liver disease, Alcohol Res. Health, 2003, 27, 307)].
Varieties of endogenous enzymatic and non-enzymatic mechanisms have evolved to protect cells against ROS. This includes the superoxide dismutases (SOD), which remove O2−; Catalase (CAT) and the glutathione peroxidase (GPX) system, which remove H2O2 and non-enzymatic low-molecular-weight antioxidants such as reduced glutathione (GSH), Vitamin E, Vitamin C, Vitamin A, Ubiquinone, Uric acid, and bilirubin. But these are capable to protect the cells to limited extent. Additional protection could be achieved by orally administrating the glutathione precursor like S-adenosyl-L-methionine (SAMe), N-acetyl cysteine (NAC) or anti-oxidant like Vitamin E. Vitamin C, plant bioactives (gallic acid, quercetinete) etc. (D. Wu, Alcohol Research & Health, 2006, 27, 4, 277).